1. Field of the Invention
Embodiments disclosed herein relate generally to the measurement of the density of a fluid in a vessel using gamma radiation. Specifically, embodiments disclosed herein relate to the measurement of the density of a fluid in a vessel by detecting the intensity of gamma rays backscattered by the fluid from a gamma-ray source.
2. Background
Gamma rays have been used to measure the density and level of fluids in a vessel by using a gamma-ray source positioned opposite a gamma-ray detector. These through-transmission gamma-ray density and level measurements are useful where the materials measured are hazardous, extremely hot, or where direct contact measurements are otherwise not possible. Additionally, the source and detector are mounted outside the vessel, and no modification to the vessel is required. Gamma rays emitted by a source may be absorbed or attenuated by the vessel and the material in the vessel. The strength of the gamma radiation reaching a detector opposite the source may be used to indicate the density or level of a fluid in a vessel based upon the intensity of the source.
When measuring fluid level, for example, multiple gamma-ray emitters and/or detectors may be positioned at opposite sides of a vessel, where the presence or absence of a signal (or a nominal low signal) may indicate the presence or absence of a fluid in place between the source and detector. The size of a vessel in a signal/no signal level detector may be much larger than that for a gamma-ray densitometer, as described below, as gamma rays are not as readily absorbed or attenuated by vapors in the vessel.
With respect to fluid density, for example, fluid passing between the gamma-ray source and detector may absorb or attenuate gamma rays emitted by the source. A high radiation count indicates a low fluid density while a low count indicates high fluid density.
Referring now to FIG. 1, one example of a prior-art through-transmission gamma-ray densitometer is illustrated. A housing (not shown) may be mounted on a tubular pipe or vessel 10 with a bore 12 which contains a fluid 13. A source of gamma radiation 14 is located on one side of the bore 12 and, a gamma radiation detector 15 is located on an opposite side. The radiation provided by the source 14 is a constant intensity over a long period of time (random intensity over a finite period) of gamma-ray emissions. The gamma rays are transmitted through the material surrounding the bore 12, the fluid 13 within the bore and to the detector 15. The detector 15 may be, for example, a crystal of sodium or cesium iodide (thallium activated) or other material capable of scintillating under irradiation and may include an electron photomultiplier tube for converting light flashes of the scintillation of the crystal into an electrical pulse.
A primary variable with respect to the amount of gamma rays emitted from source 14 that reach detector 15 is fluid 13 contained within vessel 10. A percentage of the gamma rays emitted by source 14 are absorbed or attenuated by fluid 13 and do not reach detector 15. Thus, the counting rate of the output signal from the photo multiplier tube of detector 15 may be related to the density of fluid 13 through which the rays must pass to reach detector 15 and the intensity of source 14.
However, through-transmission density measurement using gamma rays is viable only for limited vessel sizes and/or fluid densities. For example, for a similar sized source, at higher fluid densities, the fluid may absorb more gamma rays, thus resulting in fewer gamma rays reaching the detector. Similarly, as vessel size is increased, gamma rays must pass through a greater quantity of material (vessel and fluid) absorbing the gamma rays, resulting in fewer gamma rays reaching the detector. Therefore, gamma-ray density measurements in this manner are currently only viable for vessels up to about 1 meter in diameter.
Vessel thickness may also limit the effectiveness of through-transmission gamma-ray density measurements. As vessels absorb and attenuate gamma rays in a manner similar to fluids, and a higher wall thickness may result in fewer gamma rays reaching the detector. Vessel thickness may be regulated by code, such as ASME or other vessel specifications, where the required thickness may be based upon operating pressure and the nature of the fluid (corrosive, erosive, reactive, etc.). Furthermore, current safety margins for vessel thickness may increase and may further limit the effectiveness of through-transmission measurements.
Another disadvantage in the present use of gamma rays for through-transmission density measurements is that the solid angle subtended by a fixed size detector, and thus the counting rate, scales inversely with the size of the vessel squared. The counting rate n may be approximated by the equation:n˜Ωe−d/λ˜(e−d/λ)/d2  (1)where n is the counting rate, d is the vessel diameter, and λ is the absorption length which depends on density. For a similar sized detector, a lower count rate may result in a greater rate of error or may require a larger source to maintain a desired accuracy. Alternatively, as vessel size is increased, detector size may be increased to maintain a constant count rate. Regardless, increasing the size of the source and/or the size of the detector will invariably increase costs.
To overcome the thickness, size, and density limitations, the intensity of the gamma-ray source may be increased, thus resulting in a measurable quantity of gamma rays reaching the detector. However, cost, safety, multi-unit effectiveness, and security may each limit the source intensity that may be used. For example, the use of a radioactive source creates personnel safety and environmental concerns and requires lead or tungsten shielding to protect personnel, special handling precautions and equipment, as well as disposal and remediation procedures. Furthermore, because gamma rays are produced from a point source and not a directional source, as the size of the source increases, the amount of shielding required to contain the radiation in directions other than through the vessel must be increased, thus adding further to the cost.
With respect to multi-unit effectiveness, a chemical plant may desire to use gamma-ray level and density gages on multiple vessels, for example. However, as the number of gages is increased or the intensity of gamma-ray sources is increased to overcome size limitations, cross-talk between gamma-ray sources and detectors on adjacent vessels may occur, resulting in decreased effectiveness and potentially erroneous readings.
Regarding security, due to growing worldwide concerns about the proliferation and possible smuggling or other transport of radioactive nuclear materials, state, local, and national governments regulate facility security requirements based upon the total amount of radioactive material that may be present at a single site. For example, the State of Texas requires additional security measures (e.g., background checks, accessibility, etc.) at facilities where the total Curie count exceeds 27 Curie, where the total Curie count is based upon a sum of all radioactive sources at the facility. Thus, use of larger sources to overcome vessel size limitations may result in an increased need for security at an additional cost.
Accordingly, there exists a need for gamma-ray density gages that may be used on larger vessels. Additionally, there exists a need for non-contact density gages that require lower intensity radiation sources.